1. Field of the Invention
The present invention relates to a lubricant structure of an engine, particularly to a lubricant structure suitable for an engine for maintaining a dry state so that oil is not gathered in a crank chamber while driving and also utilizing a power transmission chamber in a crank case as an oil reserving or gathering portion, which is a so-called semi-dry sump type engine.
2. Description of the Prior Art
Prior Art
Unlike a dry sump type engine, there is no need for providing a separate oil tank for a semi-dry sump type engine. Therefore, it is possible to save cost and arrangement space for the oil tank. It should be noted that with another oil pan type engine, there is a problem that a lower side of the engine protrudes and hence engine height is elongated.
In general, the semi-dry sump type engine is provided with two oil pumps. One is a first oil pump for pressure-feeding oil from an oil reserving portion to a plurality of lubricant points of the engine, which is a so-called feed pump. The other is a second oil pump for pumping the oil returned from the lubricant points to a crank chamber and the like up to the oil reserving portion, which is a so-called a scavenging pump. Primary oil filters are respectively connected to oil intake or suction sides of the two oil pumps.
FIG. 15 is a conventional semi-dry sump type engine disclosed in Japanese Patent Laying-Open No. 2007-9738. An arrangement structure of primary oil filters of a feed pump and a scavenging pump in this engine will be described. An oil pan 301 which is a separate member from a crank case 300 is attached to a lower surface of the crank case 300. A funnel-shaped primary oil filter 303 communicating with an oil intake side of the feed pump (not shown) and a primary oil filter 304 communicating with an oil intake side of the scavenging pump (not shown) are arranged in this oil pan 301. It should be noted that the crank case 300 has dually divided into upper and lower members, and only the lower crank case member is schematically shown in the drawing.
In a case where the primary oil filters 303 and 304 are exchanged, after the oil pan 301 attached to a lower part of the crank case 300 by a bolt or the like (not shown) is removed, the primary oil filters 303 and 304 are removed from the oil pan 301, and then new primary oil filters or the cleaned primary oil filters are attached respectively.
In a conventional semi-dry sump type engine disclosed in Japanese Patent Laying-Open No. 1994-288466 and Japanese Patent Laying-Open No. 1996-135419, an intake portion of a scavenging pump directly communicates with a crank chamber, and oil dropping down into the crank chamber is suctioned from an oil intake port in a lower end and discharged to a power transmission chamber so as to maintain an inside of the crank chamber in a dry state.
Further, FIG. 16 shows a detailed description for oil pumps and oil passages of said Prior Art, Japanese Patent Laying-Open No. 2007-9738. However, reference numerals are refurbished. In FIG. 16, a crank case 401 is dually divided into upper and lower crank case members 401a and 401b, and separated by a division wall 402 into a crank chamber 403 of a front part and a power transmission chamber 404 of a rear part. An oil pan 405 is attached to a lower surface of the lower crank case member 401b. A second oil pump (a scavenging pump) 412 for pumping oil gathered in the crank chamber 403 up to the power transmission chamber 404 and a first oil pump (a feed pump not shown) for pressure-feeding the oil gathered in a lower part of the power transmission chamber 404 to a lubricant point are arranged in parallel in a lower end of the division wall 402.
The second oil pump 412 has an oil discharge port 412a directly opening in the power transmission chamber 404, suctions the oil gathered in a lower space portion 403a of the crank chamber 403 via a filtering instrument 413 and an oil passage 414 on an intake side, and directly discharges the oil from the oil discharge port 412a to an oil reserving portion 404a of the power transmission chamber 404.
It should be noted that a part of the oil discharged by the first oil pump is supplied to a cam shaft and a rocker arm and the like arranged in a rocker arm chamber on an upper surface of a cylinder head. In general, the oil already used in the rocker arm chamber passes through a cam chain tunnel, a generator chamber, a clutch chamber or the crank chamber, and returns to the lower space portion 403a. The oil is returned to the oil reserving portion 404a in the lower part of the power transmission chamber 404 by the second oil pump 412 together with the oil returned from other lubricant points.
Problems of Prior Arts
As a structure of a crank case of an engine, in addition to the structure divided into the upper and lower members as in FIG. 15, there is a structure divided into left and right members in a crankshaft direction, that is, a structure divided in a left and right direction. In any structure, in a case where the oil pan which is the separate member is attached to the lower surface of the crank case as in FIG. 15, it is possible to easily exchange both the primary oil filters by removing the oil pan from the crank case. However, in order to simplify and compactify the structure of the crank case, in a semi-dry sump type engine not provided with an oil pan which is a separate member, the crank case itself has to be divided into upper and lower or left and right members. It is troublesome to perform an exchange work of the primary oil filters, and hence workability at the time of exchange is lowered. Particularly, in a case where the crank case divided into the left and right members (in the crankshaft direction) is provided, the crank case cannot be split unless a crankshaft, and transmission output and input shafts are removed from at least one of the crank case members. Therefore, the workability at the time of exchange of the primary oil filters is extremely lowered.
As shown in Japanese Patent Laying-Open No. 1994-288466 and Japanese Patent Laying-Open No. 1996-135419, the structure of directly suctioning the oil from the inside of the crank chamber by the scavenging pump has the following problems.
(1) A pressure change is repeated in the crank chamber in accordance with a volumetric change by raising and lowering a piston. However, with the structure of directly suctioning the oil from the crank chamber, the pressure change in the engine influences over suctioning force of the scavenging pump, and a suctioning amount of the scavenging pump is changed. Therefore, it is not possible to sufficiently utilize a suctioning ability of the oil pump.
(2) Since the oil in a bottom part of the crank chamber is rolled up by rotation of a crank web, agitation resistance of the crankshaft is increased. Further, since bubbles are generated in the oil, air entrainment is caused in the scavenging pump, and hence pump efficiency is lowered.
In the structure shown in FIG. 16, a lot of bubbles are contained in the oil suctioned from the lower space portion 403a of the crank chamber 403 by the second oil pump 412. When the oil containing a lot of bubbles is gathered or collected in the oil reserving portion 404a of the power transmission chamber 404, in a case where the oil in the oil reserving portion 404a is suctioned by the first oil pump, the air entrainment is caused, the pump efficiency is lowered and supply efficiency of the oil to the lubricant point is also lowered. Therefore, the oil already used in the lubricant point is desirably returned to the power transmission chamber 404 without passing through the crank chamber and the second oil pump as much as possible.